This invention relates to a sealer apparatus and particularly to cross sealers and presealers for forming thermal seals of layered plastics which extend across relatively wide webs, and particularly in bag making machines.
The processing of plastic webs into bags and other like plastic members may include a relatively wide web passed through a single machine to form a plurality of bags or other members. The plastic layers in the web are sealed to each other along lines generally extended across the web by passing of the wide web through a cross sealer or a presealer having opposed seal bars spanning the web and in which the web is moved in stepped sequence. During a short seal period, the web is clamped between the seal bars to provide a thermal seal of the webs to each other. Generally, prior art cross sealers and presealers included opposed movable seal bar units which span the entire seal width of the machine. The movable seal bar structures are supported for reciprocal movement into and from engagement with each other. The movable seal bar units are secured at one end or both ends to a drive unit. A cam driven mechanism and the associated connections to the seal bars are driven by an appropriate power system such as an AC motor drive, a servo motor drive or even an air cylinder drive. The cam driven mechanism includes separate cam assemblies at each side plate of the machine with the cam assemblies secured to the opposite ends of a single rotating shaft. The respective cam assemblies are coupled to the outer ends of the seal bar and designed to move the heavy seal bar and establish an adequate seal pressure to affect the desired seal over the entire seal width of the machine. Another design has included multiple air cylinders spaced across the seal bars. The prior art machines require relatively massive seal bar structures which establish an extremely rigid bar because of the high operational seal pressures required and to produce and maintain an even pressure across the length of the seal bar. The combination of the various parts creates so much inertia that a relatively slow rate of movement is required and with a dwell control, the machine speed is significantly limited. AC motors, in particular servo motor drives, produce a better control of the sealer mechanism and particularly the movable seal bar during the start/stop sequence. The performance is not considered adequate to fully provide in-line compliant seal bar movement with a modem bag making machine. Attempts made to increase the speed of the prior art sealers resulted in significant increase in maintenance problems and associated downtimes.
A typical prior art cross sealer assembly with the wide cross seal bars often include a cloth covering applied to the seal surfaces of the seal bars to avoid attachment of the plastic webs to the sealing faces of the opposed seal bars. In such constructions, the cloth assembly is secured to the frame structure with a source roll of the cloth secured to one side of the seal bar. The cloth passes therefrom across the seal surface of the seal bar to a take-up roll which is rotatably mounted to the opposite side of the seal bar unit. Similar cloth assemblies are provided for both the upper and lower seal bar units. The cloth is typically a TEFLON material (trademark of E.I. duPont de Nemours and Company of Delaware, USA).
The wide web is often used where a plurality of laterally spaced members are formed within the single web to increase the production of product. Such machines are well known in the bag making machine for pouches and like plastic containers. A typical machine for example which had been commercially available had a web width of 1,240 millimeters. Although satisfactory bag making machines using wide cross sealers or presealers have been designed and sold, there is a need for improved cross sealers constructed to operate in a more rapid cycle time while maintaining the creation of acceptable cross seals of the plastic laminates.
The present invention is particularly directed to the design of wide plastic sealer units with a relatively compact design and in which the seal bar structure has a low inertia and is supported by a multiple of spaced mechanical linkages secured to a shaft of a common drive mechanism connected across the sealer apparatus. The rotating shaft is driven by an appropriate system, preferably a servo motor drive or a pneumatic cylinder drive, for rotating of the shaft and thereby raising and lowering of a seal bar in repetitive cycles through the spaced mechanical linkages. The multiple interconnecting mechanisms are preferably eccentric link unit or individual cam units mounted in spaced relation across the width of the sealer structure and attached to support a rigid support beam or like support structure for vertical movement. A light seal bar is secured to the support structure for movement into operative engagement with an opposed substantially rigid stationary and rigid support structure including an opposed seal bar. The multiple connecting mechanisms move the movable seal bar to the sealing position and develops a high seal pressure across the bar to seal the plastic laminates. The combination of the low inertia seal bar with the multiple linkages in combination with the opposed rigid stationary seal support structure provides a highly effective and relatively high speed sealing device. In this preferred system, only the one sealer bar is moved during the seal cycle while the other one is stationary and does not add to the inertia of the overall seal moving system. With the multiple connections, the seal bar system inertia is directly proportional to the width of the mechanism. The cross section of the seal bar remains the same and only requires an appropriate number of multiple links to maintain the uniform and high seal pressure across the seal bar unit.
In a preferred construction, a lightweight but rigid support beam is connected to the multiple linkages and connected to a seal bar by a plurality of adjustment units. The seal bar adjustment unit, such as threaded connecting studs, are staggered between the top and bottom seal bar units. This establishes a natural deflection of the fixed seal bar between the studs of the upper seal bar to offset any deflection of the lower seal bar. As a result the bars fully conform to each other and establish and assure a better seal integrity across the web width. The seal bar units are designed to permit easy variation in the width or character of the seal produced. For example, a stationary or movable seal bar may be readily formed to receive different inserts along the length of the bar. Similarly, the movable bar can be readily formed by milling or otherwise forming of different patterns in the face of the movable bar.
In the preferred construction, the movable seal bar is also designed to be connected to the rotating shaft by the different connecting mechanisms which allow for adjustment of the web seal line to the thickness of the web material being passed therethrough. Thus, bubble pack and foam webs are generally significantly thicker material and require different movement of the seal bars.
A preferred stationary seal bar unit includes a similar rigid support structure interconnected to a seal bar by a plurality of like adjustable interconnections such as adjustable studs. The system is mounted for limited reciprocal movement with a plurality of laterally spaced support structures, some of which include a air cylinder or like member, to establish and maintain the desired seal pressure. The cylinder units permit dropping of the sealer when the machine is stopped so as to prevent the continued heated engagement of the stationary seal bar with the web structure. In the latter preferred construction, the adjustment studs for the respective first and second seal bars are offset relative to each other. In the operation of the unit which requires relatively high pressures any deflection within the seal bars tend to conform to each other and maintain an optimal seal pressure across the web.
The present inventor has further discovered that the seal bars may be coated with a release coating which prevents the softened and molten plastic from adhering to the face of the hot seal bar or bars. This eliminates the special moving cloth member passed over the seal bar faces between the seal bars and the web laminates; thereby reducing the initial cost and maintenance of such mechanisms.
The present invention with the multi-connections across the seal bar unit produces an improved thermal seal with an increased speed, with cost effective result.
Various other features, objects and advantages of the invention will be made apparent from the following description taken together with the drawings.